Magnetic gradient field projection

Abstract

The invention relates to the projection of magnetic gradient fields for use in Magnetic Resonance Imaging. The projection of a gradient outside of a field generating assembly allows imaging of samples which will not physically fit inside the coil confines. An apparatus and method for improving the linearity of the projected gradient field is disclosed. This is achieved by arranging the position of the magnetic field generating coil conductors such that the second derivative of a gradient component is equal to zero. In one practical example the linearity is improved by providing a first circular coil which is positioned so that its centre is offset along the axis of the field generating coil assembly by a distance equal to half the radius of the coil. In another example a pair of current carrying wires are selectively positioned offset from a region of interest.

1 . Magnetic gradient field projection apparatus for generating a magnetic field along a projection axis, in a defined sample volume offset from and outside said apparatus, comprising gradient field generating means arranged to generate a substantially linear axial gradient component in said sample volume, said gradient field generating means comprising a substantially circular coil positioned so that the centre of the coil is displaced along the axis of the coil away from the centre of the sample volume by a distance approximately equal to half of the radius r 1  of the coil, such that the second derivative of the axial gradient component is substantially equal to zero at the centre of the sample volume.  
     
     
         2 . Apparatus according to  claim 1  comprising a further coil, the axis of the further coil being substantially coincident with the axis of said coil, and distance of said further coil from the centre of the sample volume, and the radius of the further coil, being selected such the angle subtended at the centre of the sample volume by each of said coils is substantially the same.  
     
     
         3 . Apparatus according to  claim 2 , the distance of said further coil from the centre of the sample volume being substantially equal to half of the radius r 2  of the further coil, the coils being connected so that, in use, the relation  
       
         
           
             
               
                 
                   ( 
                   
                     
                       r 
                       2 
                     
                     
                       r 
                       1 
                     
                   
                   ) 
                 
                 4 
               
               = 
               
                 - 
                 
                   
                     
                       I 
                       2 
                     
                      
                     
                       N 
                       2 
                     
                   
                   
                     
                       I 
                       1 
                     
                      
                     
                       N 
                       1 
                     
                   
                 
               
             
           
           
           
               
           
         
       
       is substantially satisfied where I 1  and I 2  are the currents flowing through said coil and said further coil respectively, and N 1  and N 2  are the numbers of turns of said coil and said further coil respectively, the radii of the coils being mutually different, the currents in the coils flowing in opposite directions.  
     
     
         4 . Apparatus according to  claim 1  comprising a plurality of substantially circular coils of differing radii arranged in substantially parallel planes with their centres passing through a common axis, the distance of each coil from the centre of the sample volume, and the radius of each coil, being selected such the angle subtended at the centre of the sample volume by each of said coils is substantially the same.  
     
     
         5 . Magnetic gradient field projection apparatus for generating a magnetic field along a projection axis, in a defined sample volume offset from and outside said apparatus, comprising gradient field-generating means arranged to generate, in use, a transverse gradient component of said magnetic field which is substantially linear within said defined sample volume, said gradient field-generating means comprising two substantially parallel long substantially straight conductors substantially symmetrically located about an axis passing through the centre of the sample volume, said conductors lying in a plane substantially perpendicular to the projection axis and offset along said projection axis from the centre of the sample volume, wherein the conductors are arranged to carry substantially equal currents in a common direction and are positioned relative to the centre of the sample volume such that the second derivative of the transverse gradient component in the transverse direction is substantially equal to zero at the centre of the sample volume.  
     
     
         6 . Apparatus according to  claim 5 , wherein the spacing a 1  between the conductors and the distance d 1  of the plane through the conductors from the centre of the sample volume, substantially satisfy the relation a 1   2 =4d 1   2 (3+/−2{square root}2).  
     
     
         7 . Apparatus according to  claim 6 , wherein a further pair of substantially parallel conductors is provided, the plane of the further pair being substantially parallel to the plane of said pair of conductors, the current in the further pair arranged to flow in the opposite direction to the current in said pair of wires, the distance of the further pair being greater than the distance of said pair of wires, the separation a 2  of the further pair of conductors or wire bundles and the distance d 2  of the further pair of conductors or wire bundles from the sample plane substantially satisfying the relation a 2   2 =4d 2   2 (3+/−2{square root}2), the currents in the conductors or wire bundles further substantially satisfying the relation ΣI 2 /ΣI 1 =−(a 2 /a 1 ) 3 , where ΣI 1 , ΣI 2  are the total currents in each conductor for the first and second pairs respectively.  
     
     
         8 . Apparatus according to  claim 6 , wherein a further pair of substantially parallel conductors is provided, the plane of the further pair being substantially parallel to the plane of said pair of conductors, the current in the further pair arranged to flow in the opposite direction to the current in said pair of wires, the distance of the further pair being greater than the distance of said pair of wires, the separation a 2  of the further pair of conductors or wire bundles and the distance d 2  of the further pair of conductors or wire bundles from the sample plane substantially satisfying the relation a 2   2 =4d 2   2 (3÷/−2{square root}2), the currents in the conductors or wire bundles further substantially satisfying the relation ΣI 2 /ΣI 1 =−(a 2 /a 1 ) 6 , where ΣI 1 , ΣI 2  are the total currents in each conductor for the first and second pairs respectively.  
     
     
         9 . A magnetic field projection apparatus comprising a plurality of gradient field generating elements arranged to generate substantially linear magnetic gradient field components in mutually orthogonal directions in a sample volume offset from and outside said apparatus, wherein said magnetic field generating elements comprise 
 an axial gradient component generating element comprising a substantially circular coil positioned so that the centre of the coil is displaced along the axis of the coil away from the centre of the sample volume by a distance approximately equal to half of the radius r 1  of the coil, such that the second derivative of the axial gradient component is substantially equal to zero at the centre of the sample volume, and;    a transverse gradient component generating element, comprising two substantially parallel long substantially straight conductors substantially symmetrically located about an axis passing through the centre of the sample volume, said conductors lying in a plane substantially perpendicular to the projection axis and offset along said projection axis from the centre of the sample volume, wherein the conductors are arranged to carry substantially equal currents in a common direction and are positioned relative to the centre of the sample volume such that the second derivative of the transverse gradient component in the transverse direction is substantially equal to zero at the centre of the sample volume.    
     
     
         10 . Apparatus according to  claim 1  further including 
 A housing;  
 Means for defining a sample volume outside the housing;  
 A magnet assembly within the housing for generating a static magnetic field within the defined sample volume along the projection axis;  
 a radio frequency transceiver within the housing for transmitting signals to and receiving signals from the defined sample volume;  
 control apparatus for controlling said magnetic field generating elements to generate said at least one gradient component in conformity with an imaging protocol.  
 
     
     
         11 . Apparatus according to  claim 10 , wherein said means for defining the sample volume comprises an operating surface to the outside of said housing over which a sample to be imaged, preferably a patient, can be positioned.  
     
     
         12 . Apparatus according to  claim 1 , further comprising means for applying current to generate desired gradient components in accordance with a given imaging protocol.  
     
     
         13 . Magnetic resonance spectroscopy system comprising an apparatus according to  claim 1 .  
     
     
         14 . A method of configuring a magnetic gradient field projection apparatus according to  claim 1  to produce a substantially linear magnetic field gradient component within the defined sample volume, comprising positioning the gradient field generating means with respect to the sample volume so that the second derivative of a desired component of the magnetic field with respect to distance along a predetermined direction corresponding to said magnetic field gradient component is substantially equal to zero at least about a predetermined joint within the defined sample volume, preferably at or adjacent the centre of the sample volume.  
     
     
         15 . A method according to  claim 14 , further comprising arranging the assembly so that the third derivative of said component of the magnetic field with respect to distance along said predetermined direction is substantially equal to zero at least about said predetermined point within the defined sample volume.